1. Field of the Invention
This invention concerns an organic electroluminescence device and, more specifically, it relates to an organic electroluminescence device which can be manufactured easily, and improved in stability and facilitate increase of area.
2. Description of the Related Art
The electroluminescence device (hereinafter also referred to as xe2x80x9cEL devicexe2x80x9d) is a self light emitting wholly solid device and since it has high visual recognizability and impact strength, a wide applicability is expected. Those using inorganic fluorescent materials are predominant at present and used generally. However, since they require an AC voltage of 200 volt or higher for driving, they involve a problem of high production cost and insufficient brightness. On the other hand, study for EL devices using organic compounds was started initially by using single crystals such as of anthracene. However, in the case of single crystals, the film thickness is as large as about 1 mm and a driving voltage of 100 volt or higher is necessary and the luminous efficacy is low. Therefore, it has been attempted to reduce the thickness by using a vapor deposition method (Thin Solid Films 94, pp.171-183 (1982)). However, in the thin film obtained by the vapor deposition method, since the driving voltage is still as high as 30V and the density of electrons-hole carriers are low in the film and the probability of generating photons by carrier recombination is low, no sufficient brightness can be obtained and they have not yet been put to practical use.
By the way, there has been reported recently an EL device of a function separation type in which a hole transporting organic low molecular weight compound and a fluorescent organic low molecular weight compound having electron transporting function are laminated successively as extremely thin films by a vacuum vapor deposition method on a transparent substrate, which can provide high brightness of 1000 cd/m2 at a low voltage of about 10 V (Appl. Phys. Lett., 51, pp. 913-915 (1987)) and, ever since, vigorous research and development have been conducted for laminate type EL devices.
However, in the EL device of this type, since thin films of 0.1 xcexcm or less are formed in plural vapor deposition steps, pinholes are liable to be formed and it is necessary to control the film thickness under strictly controlled conditions in order to obtain a sufficient performance. Accordingly, there is a problem that the productivity is low and increase of the area is difficult. Further, since the EL device is driven at a high current density of several mA/cm2, a great amount of Joule heat is generated. Therefore, a phenomenon is often observed that the hole transporting low molecular weight compound or the fluorescent organic low molecular compound formed as films by vapor deposition in an amorphous glass state is gradually crystallized and finally melted to lower the brightness or result in insulation breakdown and, as a result, the life time of the device is lowered.
Hence, there has been reported of using a star burst amine capable of providing a stable amorphous glass state as the hole transporting material (Extended Abstracts (The 40th Meeting, 1993); The Japan Society of Applied Physics, 30a-SZK-14, 1993), or using a polymer formed by introducing a triphenyl amine to the side chain of polyphosphazene (Pretext for The 42nd Symposium on Macromolecules, 20J 21 (1993)) for solving the problems in view of the heat stability of EL devices. However, since an energy barrier caused by ionizing potential of the hole transporting material is present, they cannot satisfy by themselves the hole chargeability form the anode or the hole chargeability to the light emitting layer. Further, in the former case of using the star burst amine, purification is difficult due to the low solubility and it is difficult to enhance the impurity, and the latter polymer cannot provide high current density to obtain a sufficient brightness.
On the other hand, research and development have also been progressed also on the EL devices of a single layer structure intending to solve the problems described above and devices formed by using conductive polymers such as poly(p-phenylene vinylene) (Nature Vol. 357, pp. 477-479, 1992) or mixing an electron transporting material and a fluorescent dye in hole transporting polyvinyl carbazole (Extended Abstracts (The 38th Meeting, 1991); The Japan Society of Applied Physics, 31p-G12, 1991) have been proposed but they are not yet comparable with the laminate type EL devices using the organic low molecular weight compound in view of the brightness and the luminous efficacy.
This invention has been made in view of overcoming the foregoing problems in the relatedr art, and this invention intends to provide an organic electroluminescence device easy to manufacture, having satisfactory brightness and excellent in durability.
The present inventors have made an earnest study on hole transporting polymers for solving the foregoing and, as a result, have accomplished this invention, based on the finding that a hole transporting polyester including, as a partial structure, at least one member selected from the structures shown by the following structural formulae I-(1) and I-(2) has a favorable hole charging property, a hole mobility and thin film forming performance, which is suitable as an organic electrtoluminescent device.
An aspect of the present invention provides an organic electroluminescence device having one or plural organic compound layers put between a pair of electrodes including an anode and a cathode one of which is transparent or semitransparent, wherein at least one of the organic compound layers contains one or more kinds of hole transporting polyesters including a repeating unit containing, as a partial structure, at least one member selected from structures represented by the following general formulae (I-1) and (I-2): 
where Ar represents a substituted or unsubstituted monovalent polynuclear aromatic ring having a number of aromatic rings of from 3 to 10, or a substituted or unsubstituted monovalent condensed aromatic ring having a number of aromatic rings of from 2 to 10, X represents a substituted or unsubstituted bivalent aromatic group, T represents a bivalent linear hydrocarbon group of 1 to 6 carbon atoms or a bivalent branched hydrocarbon group of 2 to 10 carbon atoms and k represents 0 or 1.
An organic electroluminescence device according to another aspect of the present invention, has a hole transporting layer containing one or more hole transporting polyesters including repeating units containing, as a partial structure, at least one member selected from structures represented by the general formulae (I-1) and (I-2), and a luminescent layer as the organic compound layer in this order on a transparent electrode.
In an organic electroluminescence device according to another aspect of the present invention, the organic compound layer is a single layer.
In an organic electroluminescence device according to another aspect of the present invention, the organic compound layer contains at least one of a luminescent material, a hole transporting material and an electron transporting material.
In an organic electroluminescence device according to another aspect of the present invention, the hole transporting polyester is a polyester represented by the following general formula (I) or (III): 
where A represents at least one member of the structures represented by the general formula (I-1) and (I-2), R represents a hydrogen atom, an alkyl group, a substituted or unsubstituted aryl group or a substituted or unsubstituted aralkyl group, Y represents a bivalent alcohol residue, Z represents a bivalent carboxylic residue, B and Bxe2x80x2 each represents, independently, xe2x80x94Oxe2x80x94(Yxe2x80x94O)mxe2x80x94R or xe2x80x94Oxe2x80x94(Yxe2x80x94Oxe2x80x94)mxe2x80x94COxe2x80x94Zxe2x80x94COxe2x80x94ORxe2x80x2 (in which R, Y and Z have, respectively, the same meanings as above and Rxe2x80x2 represents an alkyl group, a substituted or unsubstituted aryl group or a substituted or unsubstituted aralkyl group and m represents an integer of 1 to 5), m represents an integer of 1 to 5 and p represents an integer of from 5 to 5000.